Remove GPG chunk tempfiles after they're used instead of at global destruction

Active GPGProcess objects are keyed by the PositionedChunk address, not
their own address.  This bug left dangling references to GPGProcess
objects which in turn kept references to their File::Temp object,
causing any GPG-encrypted content to collect on disk until global
process destruction.  Rather than passing the pchunk to
get_proc_chunkref, simply move the delete up into the caller which
already manages $self->{procs}.

This restores the original behaviour (and current code's intent)
pre-bf78b0c.